JPH11108614A - Light-wave interference measuring instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-wave interference measuring instrument which can measure the relative displacement between a moving mirror and a fixed mirror with high accuracy, even when a plane mirror is used as the moving mirror. SOLUTION: A light from a light source section 205 is split by means of a light splitting element 93, and the light transmitted through the element 93 is made incident on a measuring section 204a. Two light beams emitted from the section 204a are reflected at a position 406 of a fixed mirror 50 and a position 407 of a moving mirror 51, and again return to the measuring section 204a. Therefore, the relative geometric displacement between the mirrors 50 and 51 can be found. On the other hand, the light reflected by the light splitting element 93 is made incident on a measuring section 204b through a reflecting mirror 45. Two light beams emitted form the section 204b are reflected at the positions 408 and 409 of the moving mirror 51 and again return to the measuring section 204b, and the inclination angle in the P-direction caused by the movement of the moving mirror 51 is found and, at the same time, the fluctuating component of refractive index between the measuring section 204b and moving mirror 51 is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light wave interference measuring device for measuring displacement with high accuracy.

[0002]

2. Description of the Related Art As a typical example of an optical interference measuring apparatus for measuring the length, displacement, density, etc. of an object, an interferometer for correcting the refractive index fluctuation of a measuring optical path for measuring displacement is shown in FIG.
Shown in The optical interference measuring apparatus shown in FIG. 7 has a displacement interferometer for measuring the displacement of an object and an optical system for measuring a change in the refractive index of gas in a measurement optical path of the displacement interferometer. First, the displacement interferometer will be described.

In FIG. 7, a light source 3 emits light including light having a frequency f1 and light having a frequency f1 '. The two lights are linearly polarized light having slightly different frequencies and mutually orthogonal polarization directions. The light beam emitted from the light source 3 is reflected by the reflecting mirror 48, reflected again by the frequency coupling element 81, and becomes coaxial with the light from the light source 1. The frequency coupling element 81 has a property of reflecting light of the frequency f1 and light of the frequency f1 ′ and transmitting light of other frequencies.

The frequencies f1 and f1 'reflected by the reflecting mirror 81 are
Is incident on the polarization separation element 70, and the light of the frequency f1 is transmitted through the polarization separation element 70 and becomes the measurement light, and the frequency f1 ′
Is reflected by the polarization separation element 70 and becomes reference light.

[0005] The frequency f1 'reflected by the polarization separation element 70
Is reflected by the reflecting mirror 40, passes through the wave plate 100, and is reflected by the plane reflecting mirror 50. The light having the frequency f1 ′, which is the reference light, travels to the polarization splitter 70 again via the wave plate 100 and the reflecting mirror 40. The light having the frequency f1 ′ traveling toward the polarization separation element 70 passes through the wave plate 100 twice so that the polarization direction is rotated by 90 °.
To the frequency separation element 80. This frequency f
The path through which the light of 1 'has passed is referred to as a reference optical path.

On the other hand, the measurement light having the frequency f1 transmitted through the polarization separation element 70 is reflected by a moving mirror 51, which is a plane reflecting mirror, through a wave plate 101. The light of the frequency f1 reflected by the moving mirror 51 is again transmitted through the wave plate 101 to the polarization separation element 7.
Go to zero. The frequency f toward the polarization separation element 70
The light of No. 1 passes through the wave plate 101 twice, so that the polarization direction is rotated by 90 °. Therefore, the light of No. 1 is reflected by the polarization splitting element 70 and the frequency f1 ′ of the reference light path reflected by the reflecting mirror 50
To the frequency separation element 80. The optical path through which the frequency f1 'passes is defined as a measurement optical path.

The coaxial light of the frequency f1 and the frequency f1 ′ is reflected by the frequency separation element 80. Frequency separation element 8
The light of the frequency f1 and the frequency f1 ′ reflected at 0 are interfered by the polarizer attached to the light receiving element 30 and then photoelectrically converted. Specifically, the polarizer is disposed at an angle of approximately 45 ° with respect to the polarization direction of the reference light and the polarization direction of the measurement light.

The light photoelectrically converted by the light receiving element 30 is output to the arithmetic unit 7 as a measurement beat signal. On the other hand, the frequency f1 and the frequency f
A part of the light 1 ′ is extracted, transmitted through the polarizer, photoelectrically converted, and output to the arithmetic unit 7 as a reference beat signal. The arithmetic unit 7 obtains the optical path length change D (f1) of the movable mirror 51 in the direction of the arrow based on the phase change of the measured beat signal with respect to the reference beat signal. However, since the refractive index fluctuation component due to the air pressure and the air temperature in the measurement optical path is superimposed on the optical path length change D (f1) obtained by the displacement interferometer in this way, this refractive index fluctuation is corrected. There is a need to.

Next, a description will be given of an optical interference measuring apparatus for correcting the refractive index fluctuation for obtaining the refractive index fluctuation. In FIG. 7, light having a frequency f2 is emitted from the light source 1, and a part of the light having the frequency f2 is converted by the frequency conversion element 10 into a frequency f3 (f3
= 2 × f2), and the light having the frequency f2 and the frequency f3 is emitted from the frequency conversion element 10 coaxially. The coaxial light of the frequency f2 and the frequency f3 passes through the frequency coupling element 81 and becomes coaxial with the light of the frequency f1 and the frequency f1 ′ which is the light of the displacement interferometer. At this time, the polarization azimuth of the light having the frequency f2 and the frequency f3 is 45 ° with respect to the light of the displacement interferometer.
It is set to face the direction of. Frequency coupling element 8
The light having the frequency f2 and the frequency f3 that has passed through 1 are incident on the polarization splitter 70, and the light having the frequency f2 and the light having the frequency f3 are separated by the polarization splitter 70 into reference light and measurement light.

The light having the frequency f2 and the light having the frequency f3 reflected by the polarization splitting element 70 pass through the reflecting mirror 40 and the wave plate 100 as reference light, and pass through the reflecting mirror 40 and the wavelength plate 100, like the light having the frequency f1 '.
The light is reflected at 0, and returns to the polarization splitting element 70 again. The light of the frequency f2 and the light of the frequency f3 that have passed through the polarization splitter 70 are reflected by the fixed mirror 51 as the measurement light through the wavelength plate 101 in the same manner as the light of the frequency f1, and travel toward the polarization splitter 70 again.

The lights of frequencies f2 and f3 passing through the reference light path and the lights of frequencies f2 and f3 passing through the measuring light path are made coaxial by the polarization separation element 70, and then transmitted through the frequency separation element 80 to displace interference. It is separated from the light of the frequencies f1 and f1 'of the meter.

The lights of frequencies f2 and f3 passing through the reference light path transmitted through the frequency separation element 80 and the lights of frequencies f2 and f3 passing through the measurement light path are separated again by the polarization separation element 71 into reference light and measurement light. Is done. The light of frequencies f2 and f3 reflected by the polarization splitting element 71, that is, the light of frequencies f2 and f3 passing through the reference optical path enters the frequency conversion element 11,
Light of frequencies f2 and f3 transmitted through the frequency separation element 71,
That is, the lights of frequencies f2 and f3 that have passed through the measurement optical path enter the frequency conversion element 12. Frequency conversion element 11, 1
In 2, the light of the low frequency f2 is harmonic-converted into the light of the high frequency f3, and the light of the high frequency f3 is transmitted as it is.

In this manner, the light of the frequency f3 whose frequency has been converted by the frequency conversion elements 11 and 12 first interferes with the light of the frequency f3 whose frequency has been converted by the frequency conversion element 10, and then each interference light is converted to the light receiving element 31. And 32 are respectively photoelectrically converted.

In the light receiving element 31, an interference signal is generated by the light having the frequency f2 and the light having the frequency f3 passing through the reference light path.
In the light receiving element 32, an interference signal is generated by the light of frequency f2 and the light of frequency 3 passing through the measurement optical path. Light receiving element 3
1 and the interference signal from the light receiving element 32 are both supplied to the arithmetic unit 7.

In the arithmetic unit 7, the phase difference between the interference signal due to the light having the frequency f2 and the light having the frequency f3 passing through the reference light path and the interference signal due to the light having the frequency f2 and the light having the frequency f3 passing through the measurement light path. {D (f3) -D (f2)}, which is the difference between the optical path length change D (f3) for the light of frequency f3 and the optical path length change (f2) for the light of frequency f2.
Ask for.

The arithmetic unit 7 further calculates the optical path length change D (f1) of the movable mirror 51 measured by the displacement interferometer as ｛D (f
3) The amount of change in the refractive index is corrected by an equation using -D (f2)}, and the true displacement (geometric distance) D is obtained. Less than,
The correction from the optical path length change D (f1) of the movable mirror 51 to the true displacement amount (geometric distance) D will be described. Frequency f
Optical path length change D for light of 1, f2 and frequency f3
(F1), D (f2) and D (f3) are represented by the following (Equation 1) to (Equation 3), respectively.

D (f1) = {1 + NF (f1)} × D (Formula 1) D (f2) = {1 + NF (f2)} × D (Formula 2) D (f3 ) = {1 + NF (f3)} × D (formula 3)

Here, D is a geometric distance, and N is the density of air. Further, F (f) is the frequency f of light without depending on the density of air if the composition ratio of air is unchanged.
It depends only on the function. (Equation 1) to (Equation 3)
Thus, the geometric distance D is given by the following (Equation 4).

D = D (f1) −A · {D (f3) −D (f2)} (Equation 4) where A = F (f1) / (F (f3) −F (f2)) It is.

[0020]

As described above, the conventional light wave interference measuring apparatus can measure the displacement of the movable mirror in which the refractive index fluctuation due to the fluctuation of air or the like is corrected. However, when the movable mirror moves, a change such as a tilt of the reflection surface of the movable mirror may occur for some reason. As a result, the distance actually moved by the movable mirror may differ from the measurement result obtained by the light wave interferometer.

Accordingly, an object of the present invention is to provide a device capable of measuring a displacement component of a movable mirror which is an error factor in order to minimize an error in displacement measurement of the movable mirror, and a highly accurate measurement of displacement of the movable mirror. It is an object of the present invention to provide a light wave interference measurement device that can perform the measurement.

[0022]

SUMMARY OF THE INVENTION It is an object of the present invention to measure a light beam for measuring a displacement of an object and a fluctuation of a refractive index of a gas on an optical path of the light beam for measuring a refractive index. A light source unit for emitting a light beam is provided. And it is provided in the common optical path of the measuring beam and the refractive index fluctuation measuring beam,
A first light beam splitting optical element that splits at least the length measuring light beam into a first measurement light path and a first reference light path is provided, and further a first reference light beam provided on the first reference light path. An optical system of an interferometer including a reflecting mirror and a movable reflecting mirror movably provided along at least the first measurement optical path is configured.

A first light receiving section for receiving interference light of the measuring beam having passed through the first reference optical path and the measuring beam having passed through the first measuring optical path, and at least the first measuring optical path is provided. A second light receiving unit that receives the interference light generated by the passed refractive index fluctuation measurement light beam, and detects the displacement of the movable mirror indicated by the interferometer by receiving the interference light.

Further, a second light beam splitting optical element for splitting the length measuring light beam and the refractive index fluctuation measuring light beam emitted from the light source unit into a plurality of light beams, and a light beam split by the second light beam splitting optical element. A third measurement optical path provided in one optical path and divided into a second measurement optical path from a position different from the first measurement optical path toward the movable reflecting mirror in parallel with the first measurement optical path, and a second reference optical path; The optical system has a second interferometer optical system including a light beam splitting optical element and a second reference light reflecting mirror provided on the second reference light path, and has passed through the second reference light path. A third light-receiving unit for receiving interference light of the length-measuring light beam and the length-measuring light beam that has passed through the second measuring light path, and interference generated by at least a refractive-index variation measuring light beam that has passed through the second measuring light path; And a fourth light receiving unit for receiving light.

Thus, for the same movable reflecting mirror,
Interferometers for measuring displacements at different positions are provided, and these interferometers use light emitted from the same light source. In addition, the number of the second light beam splitting elements may be plural, and the positions for measuring the displacement on the movable reflecting mirror are not necessarily limited to two. Then, the displacement of the movable mirror is measured with high accuracy by measuring the displacement of the movable reflector based on signals obtained from the first to fourth light receiving units.

In the above-mentioned light wave interference measuring apparatus, the first light beam splitter is provided in another light path of the light beam split by the second light beam splitting optical element, and the first light beam is measured from a position different from the first and second measurement light paths. A third measurement optical path parallel to the optical path toward the movable reflecting mirror, a fourth light beam splitting optical element for splitting into a third reference optical path, and a third reference light path provided on the third reference optical path. Forming an interferometer portion with the reflecting mirror and providing a fifth light receiving section for receiving interference light of the length measuring light beam passing through the third reference light path and the length measuring light beam passing through the third measuring light path. hand,
High-precision displacement measurement of the moving mirror may be performed by measuring the displacement amount of the moving reflecting mirror based on signals obtained from each of the first to fifth light receiving units. In addition, in the case of displacement measurement at a position where displacement measurement is not strictly required on the order of several nanometers, displacement measurement using a plurality of wavelengths may not be performed. The order of several nanometers differs depending on the measurement environment and the experimental system. By doing so, for example, an optical element whose wavelength characteristic is only partially good can be applied to the fourth light beam splitting optical element, so that the range of component selection is widened and the cost is reduced.

The light source unit emits light of at least two or more frequencies. By doing so, the change in the refractive index in the optical path is detected. In addition, the object is to provide a light source that emits a plurality of lights of different frequencies, and a first light source out of the light emitted from the light source.
And a first splitting optical element for splitting the light having the frequency of (1) into a measuring optical path provided with a movable mirror and a reference optical path provided with a fixed mirror, and at least the measuring optical path further includes a light different from the first frequency. And an interferometer for measuring the displacement of the movable mirror while correcting the refractive index fluctuation in the optical path, wherein a plurality of interferometers are provided, and one of the plurality of interferometers is provided. In the position where light passes through the interferometer
An interferometer different from one of the plurality of interferometers is arranged on the emission side of the light split by the second split optical element. Achieved by By providing an element for splitting a light beam in the optical path of a certain interferometer and newly providing an interferometer having the same configuration on the exit side of the split light, a plurality of light sources can be shared using the same light source. Emits light into the optical path of the interferometer and detects the displacement of multiple moving mirrors

According to the present invention, not only the refractive index fluctuation in the measurement optical path is measured and corrected, but also the pitching and yawing of the moving mirror which is a plane mirror is corrected, and the refraction of the measurement optical path for pitching and yawing correction is corrected. It is possible to realize a light wave interference measurement device that can also perform rate fluctuation correction.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical interference measuring apparatus according to a first embodiment of the present invention will be described with reference to FIGS.
The light wave interference measurement apparatus according to the present embodiment is an interference measurement apparatus using a homodyne interference method, and a schematic configuration thereof will be described with reference to FIG.

Light having a frequency f2 is emitted from the light source 1, and a part of the light is emitted by the frequency conversion element 10 to a frequency f3 (f3 = 2 ×
The light of frequency f2) is converted into light of frequency f2, and light of frequency f2 and frequency f3 is emitted from the frequency conversion element 10 coaxially. The polarization directions of the coaxial frequencies f2 and f3 are set so as to be oriented at approximately 45 ° with respect to the incident surface of the polarization separation element 70 arranged on the emission side of the frequency conversion element. A part of the light of the frequency f2 and the frequency f3 emitted from the frequency conversion element 10 is reflected by the reflecting mirror 92 and is reflected by the light flux 30.
The light propagates through the optical system shown in FIG. Although the optical system shown in FIG. 2 is not shown in FIG. 1 for simplicity, the light beam 301 is incident on the movable mirror 51 shown in FIG. 1 as shown in FIG.

The light having the frequency f2 and the light having the frequency f3 transmitted through the reflecting mirror 92 are incident on the polarization splitting element 70 and have the frequency f.
2 and the light of frequency f3, respectively,
0 separates the light into reference light and measurement light.

The light of frequency f2 and the light of frequency f3 reflected by the polarization splitting element 70 pass through the reflecting mirror 40 and the wave plate 100 as reference light, are reflected by the fixed mirror 50 as a plane mirror, and travel toward the polarization splitting element 70 again. . Note that this wave plate 100
Twice, the polarization direction of the light having the frequency f2 and the frequency f3 changes by 90 degrees. Then, the polarization separation element 70
The light having the frequency f2 and the light having the frequency f3 transmitted through the wavelength plate 101 pass through the wavelength plate 101 as the measurement light, are reflected by the movable mirror 51 of the plane mirror, and travel toward the polarization splitter 70 again.

The lights of frequencies f2 and f3 passing through the reference light path and the lights of frequencies f2 and f3 passing through the measurement light path are made coaxial by the polarization separating element 70, and then are transmitted through the reference light path.
The light of frequencies f2 and f3 passing through the measurement optical path and the light of frequencies f2 and f3 enter the measurement unit 104. The measurement unit 104 includes a frequency separation element 81 that reflects only a part of the light of the frequency f2 and transmits light of another frequency, and a light receiving element 36 that receives the light reflected by the frequency separation element 81 by causing interference. And the frequency f2 passing through the measurement optical path transmitted through the frequency separation element 81,
Frequency f2, f3 that reflects light of f3 and passes through the reference optical path
Polarization separating element 71 that transmits light of
The frequency conversion element 11 converts the light of the frequency f2 reflected by the frequency f3 into the same frequency as the light of the frequency f3, and the light of the frequency f2 converted by the frequency conversion element 11 and the frequency f3 reflected by the polarization separation element 71. A light receiving element 31 for receiving light by interfering with light; a frequency conversion element 12 for converting the light of frequency f2 transmitted through the polarization separation element 71 to the same frequency as the light of frequency f3;
Light-receiving element 3 for receiving the light of frequency f2 converted by the above and the light of frequency f3 transmitted through polarization separating element 71 by interfering with each other
2 constitutes an optical system.

By the way, of the light incident on the measuring section 104, part of the light of the frequency F2 passing through the reference optical path and part of the light of the frequency F2 passing through the measuring optical path are reflected by the frequency separation element 81, and the displacement interferometer is operated. It is configured and photoelectrically converted by the light receiving element 36. The light receiving element 36 uses homodyne interference, converts the intensity change of the interference light into an electric signal, and outputs the electric signal to the arithmetic unit 5. The arithmetic unit 5 obtains the optical path length change D (f2) of the movable mirror 51 in the direction of the arrow based on the phase change of the electric signal of the homodyne interference.

The lights of frequencies f2 and f3 passing through the reference light path transmitted through the frequency separation element 81 and the lights of frequencies f2 and f3 passing through the measurement light path are separated again by the polarization separation element 71 into reference light and measurement light. Is done. The light of frequencies f2 and f3 reflected by the polarization splitting element 71, that is, the light of frequencies f2 and f3 passing through the reference optical path enters the frequency conversion element 11,
Light of frequencies f2 and f3 transmitted through the frequency separation element 71,
That is, the lights of the frequencies f2 and f3 that have passed through the measurement optical path enter the frequency conversion element 12. Frequency conversion element 11, 1
2, the light of the low frequency f2 is harmonic-converted into the light of the high frequency f3, and the frequency f
The light of No. 3 is transmitted as it is.

In this manner, the light of the frequency f3 whose frequency has been converted by the frequency conversion elements 11 and 12 first interferes with the light of the frequency f3 whose frequency has been converted by the frequency conversion element 10, and then each interference light is converted to the light receiving element 31. And 32 are respectively photoelectrically converted.

In the light receiving element 31, an interference signal is generated by the light having the frequency f2 and the light having the frequency f3 passing through the reference light path.
In the light receiving element 32, an interference signal is generated by the light having the frequency f2 and the light having the frequency f3 passing through the measurement optical path. Light receiving element 3
1 and the interference signal from the light receiving element 32 are both supplied to the arithmetic unit 5.

In the arithmetic unit 5, the phase difference between the interference signal caused by the light having the frequency f2 and the light having the frequency f3 passing through the reference light path and the interference signal caused by the light having the frequency f2 and the light having the frequency f3 passing through the measurement light path. {D (f3) -D (f2)}, which is the difference between the optical path length change D (f3) for the light of frequency f3 and the optical path length change (f2) for the light of frequency f2.
Ask for.

The arithmetic unit 5 further calculates the displacement D (f2) of the movable mirror 51 measured by the displacement interferometer as ｛D (f3) -D
(F2) The refractive index fluctuation is corrected using｝, and the true displacement amount (geometric distance) D is obtained. Hereinafter, the correction from the displacement amount D (f2) of the movable mirror 51 to the true displacement amount (geometric distance) D will be described. Frequency f2 and frequency f3
Path length changes D (f2) and D (f3) for the light of
Is represented by the following (Equation 5) and (Equation 6), respectively.

D (f2) = {1 + NF (f2)} × D (Equation 5) D (f3) = {1 + NF (f3)} × D (Equation 6)

Where D is the geometric distance and N is the density of air. Further, F (f) is the frequency f of light without depending on the density of air if the composition ratio of air is unchanged.
It depends only on the function. From the above (Equation 5) and (Equation 6), the geometric distance D is given by the following (Equation 7).

D = D (f2) −A · {D (f3) −D (f2)} (Equation 7) where A = F (f2) / (F (f3) −F (f2)) It is.

Next, an optical system for detecting the amount of fluctuation of the movable mirror 51 will be described with reference to FIG. Light flux 3 in FIG.
01 is the frequency f reflected by the light separating element 92 shown in FIG.
2 and light of frequency f3. The light beam 30 of FIG.
Reference numeral 0 denotes the measurement light transmitted through the wave plate 101 in FIG. As shown in FIG. 2, the light beam 301 is
The light is divided into three light beams by 0 and 91. Light separating element 9
The light of frequency f2 and frequency f3 reflected by
The displacement at the position 402 of the movable mirror 51 can be measured by the interference unit 200, the interference unit 200, and the measurement unit 133. Here, the interference unit 200 includes a polarization splitting element, a reflecting mirror, and a phase plate. The polarization splitting element provided in the interference unit 200 has an incident plane of 45 degrees with respect to the polarization direction of the light beam 301. It is installed in. The phase plate provided in the interference unit 200 is disposed in the optical path between the reflection mirror and the polarization separation element provided in the interference unit 200, and the phase plate in the interference unit 200 is not reflected by the reflection plate. A device in which the polarization direction changes by 90 degrees when light makes one round trip through the mirror is used. In addition, a phase plate having the same function is also arranged between the movable mirror 51 and the polarization splitting element in the interference unit 200.

A part of the light that has entered the interference unit 200 having such a configuration is transmitted and travels to the movable mirror 51, and the other light is reflected and propagates to the reflection mirror provided in the interference unit 200. Then, the light reflected by the movable mirror 51 and the interference unit 200
The light reflected by the reflecting mirrors provided therein coaxially exits the interference unit 200 and enters the measuring unit 133. Based on the signal detected by the measurement unit, the displacement at the position 402 can be measured by the arithmetic unit 7. The measuring section 133 has the same configuration as the measuring section 104 shown in FIG.
The signal obtained by the measuring unit 133 having such a configuration is connected so as to be output to the arithmetic unit 7.

Similarly, the frequency f transmitted through the light separating element 91
Light of frequency 2 and frequency f3 is incident on an interference unit 201 having the same configuration as that of the interference unit 200. Then, the interference unit 201
The light of frequency f2 and frequency f3 emitted from the
The light is received at 4. In this way, the arithmetic unit 7 performs an arithmetic operation based on the signal obtained from the measuring unit 134, whereby the displacement at the position 403 on the movable mirror 51 can be measured.

Similarly, the light of the frequency f2 and the frequency f3 reflected by the light separating element 90 and further reflected by the reflecting mirror 41 are the same as those of the interference unit 202 and the measuring unit 133 having the same configuration as the interference unit 200. The displacement at the position 404 on the movable mirror 51 can be measured by the measuring unit 135 having the configuration described above and the arithmetic unit 7 which performs a predetermined operation based on the signal output from the measuring unit 135.

From the operation result by the operation device 7, the moving mirror 5
2, the rotation angle in the ρ direction of the coordinate system of FIG.
Since the rotation angle in the φ direction of the coordinate system in FIG. 2 can be determined from the difference between the displacements of FIG. 4 and FIG. 4, by correcting the change corresponding to this angle, the length measurement error due to the inclination of the movable mirror 51 can be eliminated. In the present embodiment, the calculation of the rotation angle in the ρ direction and the rotation angle in the φ direction is performed by the arithmetic unit 7.

Depending on the type of the device on which the optical interference measuring device is mounted, there are positions where it is not necessary to measure the displacement with an accuracy of several nanometers. When such a position is measured by the light wave interference measurement device, homodyne interference may be performed only on light of one frequency, and the result may be received to perform displacement measurement.

Specifically, for example, when it is not necessary to improve the measurement accuracy of the pitching which is the displacement in the ρ direction with respect to the yawing which is the displacement in the φ direction, the position on the movable mirror 51 shown in FIG. Regarding the measurement unit 133 for measuring the displacement of 402, the polarization separation element 71, the frequency conversion element 11,
2, the frequency separation element 81 without the light receiving elements 31 and 32
And the light receiving element 36 alone.

As described above, since the number of optical elements of the interferometer for performing measurement at a position where high-precision displacement measurement is not required can be reduced, a compact and inexpensive optical interference measuring apparatus can be realized. In addition, when displacement is measured using only one frequency of light, an inexpensive one-wavelength-compatible element can be used as the polarization splitting element in the interference section 200, so that the cost can be further reduced. Note that, instead of the frequency separating element 81, a band-pass filter that transmits only light of the frequency f2 may be used, and the light receiving element 36 may be arranged on the emission side of this filter.

Here, the light used for the length measurement by the displacement interferometer is the light having the frequency f2, but the light having the frequency f3 does not cause any problem. As described above, in the present embodiment, the light wave interference measurement apparatus capable of measuring the refractive index fluctuation of the measurement optical path together with the displacement measurement, further measures the angle fluctuation due to the movement of the movable mirror without being affected by the refractive index fluctuation, Based on the measurement results, a light wave interferometer capable of measuring the displacement of the movable mirror with high accuracy was presented.

Next, an optical interference measuring apparatus according to a second embodiment of the present invention will be described with reference to FIGS. The light wave interference measurement apparatus according to the present embodiment has an optical system that measures displacement by heterodyne interferometry and refractive index fluctuation of a measurement optical path, and also measures an angle change of a moving mirror. It is characterized in that the refractive index fluctuation of the optical path for measuring can be measured and corrected. FIG. 3 schematically shows a configuration for performing displacement measurement of the optical interference measurement apparatus according to the present embodiment and measurement of refractive index fluctuation of a measurement optical path. FIG. 4 shows an optical system for measuring the pitching component of the moving mirror and simultaneously measuring the fluctuation of the refractive index of air in the measurement optical path of the pitching measurement. FIG.
2 shows an optical system that measures the yaw component of the movable mirror and simultaneously measures the change in the refractive index of air in the measurement optical path of the yaw measurement.

First, the schematic configuration of the optical interference measuring apparatus according to the present embodiment will be described with reference to FIG. The light source 2 emits light of frequency f2 and frequency f3 (f3 = 2 × f2). The light of frequency f2 emitted from the light source 2 is
After the polarization direction is rotated by 2, the polarization separation element 7
2 separates the light into two light paths. In the wavelength plate 102, the division ratio in the polarization splitting element 72 can be changed.
The light reflected by the polarization separating element 72 is reflected by the reflecting mirror 60, enters the acousto-optic element 22, is frequency-modulated, and has a frequency f
2 '', and the light transmitted through the polarization splitting element 72 enters the acousto-optic element 21 and is frequency-modulated to a frequency f
2 'light. Light having the frequency f2 'is reflected by the reflecting mirror 61 and travels to the polarization coupling element 73. Here, although not shown, wavelengths are respectively assigned to a path from the polarization separation element 72 to the polarization coupling element 73 via the reflection mirror 61 and a path from the polarization separation element 72 to the polarization coupling element 73 via the reflection mirror 60. The plate has been inserted. Light having the frequency f2 ′ and the frequency f2 ″ is emitted coaxially from the polarization coupling element 73 and passes through the frequency coupling element 81.

On the other hand, the light having the frequency f3 emitted from the light source 2 is rotated by the wave plate 103 in the polarization direction.
The light is split into two optical paths by the polarization splitting element 74. In the wavelength plate 103, the division ratio in the polarization splitting element 74 can be changed. The light reflected by the polarization separation element 74 is reflected by the reflection mirror 62 and is incident on the acousto-optic element 23 to be frequency-modulated to become light of a frequency f3 ″. The incident light is frequency-modulated to become light of frequency f3 '. The light having the frequency f3 ″ is reflected by the reflecting mirror 63 and travels to the polarization coupling element 75. Here, although not shown, the reflection mirror 6 is
The path to the polarization coupling element 75 via the second and 63 and the polarization coupling element 7 from the polarization separation element 74 via the acousto-optic element 20
A wave plate is inserted in each of the paths to 5. The light of the frequency f3 ′ and the frequency f3 ″ exits coaxially from the polarization coupling element 75, is reflected by the reflecting mirror 43, and travels toward the frequency coupling element 81.
It becomes coaxial with the light of f2 ''. Note that this frequency f2 ',
f2 ″ and frequencies f3 ′ and f3 ″,

F3 '≠ 2 · f2 ’f3 ″ ≠ 2 · f2 ″ f3 ″ −2 · f2 ″ = f3′−2 · f2 ′

There is the following relationship. Thereafter, the components from the light source 2 to the frequency coupling element 81 shown in FIG.
Let's call it 5.

These frequencies f emitted from the light source unit 205
Lights of 2 ′, f2 ″ and frequencies f3 ′, f3 ″ are separated by the light separating element 93 and are measured by the measuring units 204a, 204b,
04c. The light separating element 93 is an optical element that can split light of all frequencies into two.

Next, the measuring section 204a measures the displacement of the movable mirror 51 and the fluctuation of the refractive index of the air in the measuring optical path, and the measuring sections 204b and 204c respectively measure the moving mirror 5
The measurement of the pitching and yawing components of No. 1 and the measurement of the fluctuation of the refractive index of air in the measurement optical path are performed. Since the configurations of the measurement units 204a, 204b, and 204c are the same, only the configuration of the measurement unit 204a is shown in FIG. 3, and the other measurement units 204b and 204c are not shown.

In FIG. 3, the light having the frequencies f2 ′ and f2 ″ and the frequencies f3 ′ and f3 ″ transmitted through the light separation element 93 is incident on the polarization separation element 70 of the measuring section 204a, Separate into reference light and measurement light.

The light having the frequency f 2 ″ and the light having the frequency f 3 ″ are reflected by the polarization separating element 70 and are used as reference light by the reflecting mirror 44.
Then, the light passes through the wave plate 100 and is reflected by the fixed mirror 50 as a plane mirror, and then travels to the polarization splitter 70 again. Polarization separation element 70
The light having the frequency f2 'and the light having the frequency f3' transmitted through the wavelength plate 101 pass through the wavelength plate 101 as measurement light,
The light is reflected at 1 and goes to the polarization splitting element 70 again.

The frequencies f2 ″ and f2 passing through the reference optical path
After the light of 3 ″ and the light of frequencies f2 ′ and f3 ′ passing through the measurement optical path are made coaxial by the polarization separating element 70, the light of frequency f2 ″ passing through the reference optical path and the frequency f passing through the measurement optical path
A part of the 2 ′ light is reflected by the frequency separation element 80 to constitute a displacement interferometer and is photoelectrically converted by the light receiving element 30. The light photoelectrically converted by the light receiving element 30 is subjected to heterodyne interference and output to the arithmetic unit 6 as an electric signal. The arithmetic unit 6 obtains a change D (f2) in the optical path length of the movable mirror 51 in the direction of the arrow based on the phase change of the electric signal of the heterodyne interference.

The light of frequencies f 2 ″ and f 3 ″ passing through the reference light path transmitted through the frequency separation element 80 and the light of frequencies f 2 ′ and f 3 ′ passing through the measurement light path are referenced again by the polarization separation element 71. Light and measurement light. The light having the frequencies f2 ″ and f3 ″ reflected by the polarization splitting element 71, that is, the light having passed through the reference optical path, enters the frequency conversion element 11, and the light having the low frequency f2 ″ is harmonic-converted to the frequency. It becomes light of 2 · f2 ″, and light of frequency f3 ″ is transmitted as it is. The light of frequency 2 · f2 ″ and the light of frequency f3 ″ are incident on the light receiving element 31 and are photoelectrically converted. The light photoelectrically converted by the light receiving element 31 undergoes heterodyne interference and is output to the arithmetic unit 6 as an electric signal. The arithmetic unit 6 determines a change in the optical path length due to a change in the refractive index of air in the reference optical path based on the phase change of the electric signal of the heterodyne interference.

On the other hand, the light having the frequencies f 2 ′ and f 3 ′ transmitted through the frequency separation element 71, that is, the light having passed through the measurement optical path, enters the frequency conversion element 12, and has a low frequency f
The light of 2 'is harmonic-converted into light of 2 · f2', and the light of frequency f3 'is transmitted as it is. Frequency 2 · f2 '
And the light of frequency f3 'enter the light receiving element 32 and are photoelectrically converted. The light photoelectrically converted by the light receiving element 32 undergoes heterodyne interference and is output to the arithmetic unit 6 as an electric signal. The arithmetic unit 6 obtains a change in the optical path length due to a change in the refractive index of air in the length measuring optical path based on the phase change of the electric signal of the heterodyne interference.

The arithmetic unit 6 corrects the displacement of the movable mirror 51 measured by the displacement interferometer based on the change in the optical path length due to the difference in the refractive index fluctuation of the air between the reference optical path and the measurement optical path, and corrects the true displacement. (Geometric distance) D is obtained. In this manner, the moving displacement amount of the movable mirror 51 can be obtained by the measuring unit 204a.

The displacement measurement of the movable mirror 51 by the measuring section 204a has been described above, but different displacement components of the movable mirror 51 can be measured by the measuring sections 204b and 204c which are completely the same.

Next, referring to FIG. 4, an optical system for measuring the pitching of the movable mirror 51 (the inclination angle of the movable mirror 51 in the ρ direction) and measuring the fluctuation of the refractive index of the measurement optical path in the present embodiment will be described. Will be described. FIG. 4 is a perspective view showing a configuration for measuring the displacement and pitching of the movable mirror 51 and measuring the fluctuation of the refractive index of air in the measurement optical path. Light of a plurality of frequencies emitted from the light source unit 205 is separated by the light separating element 93, and light transmitted through the light separating element 93 is incident on the measuring unit 204a. The two lights emitted from the measuring unit 204a are located at the position 4 of the fixed mirror 50.
06 and the position 407 of the movable mirror 51, and again the measuring unit 2
By returning to 04a, the relative geometric displacement between the fixed mirror 50 and the movable mirror 51 can be obtained.

On the other hand, the light reflected by the light separating element 93 enters the measuring section 204b via the reflecting mirror 45. Measuring unit 20
The two lights emitted from 4b are at the position 408 of the movable mirror 51.
Is reflected at the position 409 of the movable mirror 51 and returns to the measuring section 204b again. Then, the light of frequency f2 ″ reflected at position 408 and the frequency f reflected at position 409
Heterodyne interference is caused by the light 2 ′, and the light is received by the light receiving element 30 in the measurement unit 204b. In addition, interference light is received for light of other frequencies in the same manner as the measurement unit 204a. The signal obtained by each light receiving element is
Is input to the same arithmetic device as that of, and performs the same arithmetic as the arithmetic device 6 of the measuring unit 204a. The inclination angle in the ρ direction caused by the movement of the movable mirror 51 is obtained from the displacement amount obtained through such an operation, and the refractive index fluctuation component between the measuring unit 204b and the movable mirror 51 is measured. Angle measurement can be performed with the refractive index fluctuation corrected.

Based on the signal output by the arithmetic unit of the measuring unit 204a, the arithmetic unit 6 in the measuring unit 204a uses the signal output by the arithmetic unit of the measuring unit 204b to measure the displacement obtained by the measuring unit 204a. The data is transferred to the measurement unit 204
By performing correction based on the angle measurement data obtained from b, the displacement amount of the movable mirror 51 can be obtained with higher accuracy.

Next, referring to FIG. 5, an optical system for measuring the yawing of the movable mirror 51 (the inclination angle of the movable mirror 51 in the φ direction) and measuring the refractive index fluctuation of the measurement optical path in the present embodiment. Will be described. FIG. 5 is a perspective view showing a configuration for measuring the displacement and yawing of the movable mirror 51 and measuring the fluctuation of the refractive index of air in the measurement optical path. Light of a plurality of frequencies emitted from the light source unit 205 is separated by the light separating element 93, and light transmitted through the light separating element 93 is incident on the measuring unit 204a. The two lights emitted from the measuring unit 204a are located at the position 4 of the fixed mirror 50.
06 and the position 407 of the movable mirror 51, and again the measuring unit 2
By returning to 04a, the relative geometric displacement between the fixed mirror 50 and the movable mirror 51 can be obtained.

On the other hand, the light reflected by the light separating element 93 enters the measuring section 204c via the reflecting mirror 45. Measuring unit 20
The two lights emitted from 4c are located at the position 410 of the movable mirror 51.
Is reflected at the position 411 of the movable mirror 51 and returns to the measuring section 204c again. Then, the measuring unit 204 described above
The measurement unit 204c having the same configuration as that of FIG. a performs light wave interference with the light reflected at the position 410 and the position 411 of the movable mirror 51, and outputs each signal from three light receiving elements in the measurement unit 204c to the inside of the measurement unit 204c. Can be obtained by the same calculation as the calculation device in the measuring section 204b by the calculation device in the φ direction caused by the movement of the movable mirror 51.

The displacement measurement data in the measuring section 204a is output by the measuring section 204c based on the output result from the arithmetic unit in the measuring section 204a and the output result from the circular device in the measuring section 204c. By correcting with the measured angle measurement data, the displacement amount of the movable mirror 51 can be obtained with higher accuracy. Correction of the displacement measurement data based on the angle measurement data is, in the present embodiment,
The measurement is performed in the measurement unit 204a, and the output signal of the measurement unit 204c is wired so as to be input to the arithmetic unit of the measurement unit 204a.

However, the present invention is not limited to this. For the correction of the displacement measurement data, another arithmetic unit is provided and the output signal from the measuring unit 204a and the measuring unit 2 are provided in the arithmetic unit.
The wiring may be performed so that the output signal from the terminal 04c is set. In the case where the displacement measurement data is corrected by measuring the pitching of the movable mirror 51 described above, it may be performed by providing another arithmetic unit in the same manner.

By the way, the number of the light wave interference measuring apparatus according to the second embodiment of the present invention is not necessarily limited depending on the type of the apparatus on which the light wave interference measuring apparatus is mounted, as in the case of the first embodiment. There will be positions where it is not necessary to measure displacement with nanometer accuracy. When such a position is measured by a light wave interference measurement device, heterodyne interference may be performed only with light of a set of frequencies, and the result may be received to perform displacement measurement.

Specifically, for example, when it is not necessary to improve the measurement accuracy of the yawing which is the displacement in the φ direction, the relative displacement of the positions 410 and 411 on the movable mirror 51 shown in FIG. The measuring section 204c to be measured includes the polarization separating element 71, the frequency converting elements 11 and 12, the light receiving element 3
1 and 32, the polarization separating element 70 and the phase plate 10
0, 101, the frequency separating element 81 and the light receiving element 36 alone.

As described above, since the number of optical elements of the interferometer for performing measurement at a position where high-precision displacement measurement is not required can be reduced, a compact and inexpensive optical interference measuring apparatus can be realized. In addition, when measuring displacement with only one set of light waves, the interference unit 200
As for the polarization separation element, an inexpensive element for one wavelength can be used, so that the cost can be further reduced. Note that, instead of the frequency separating element 81, a band-pass filter that transmits only light of the frequency f2 may be used, and the light receiving element 36 may be arranged on the emission side of this filter.

By combining the configurations shown in FIGS. 4 and 5 described above, it is also possible to simultaneously measure all the fluctuation displacements of the movable mirror 51 for length measurement, pitching and yawing.

Next, an optical interference measuring apparatus according to a third embodiment of the present invention will be described with reference to FIG. FIG. 6 schematically shows a configuration of the lightwave interference measuring apparatus according to the present embodiment. This shows a case where a movable part such as a stage is displaced in two axes, and a stage 110 and a reference mirror (barrel) 1
11 shows an optical system in the case of measuring a displacement relative to 11 with high accuracy. In the present embodiment, measurement section 204
d and 204e are used, and their configurations and functions are shown in FIG.
Since it is the same as the measuring section 204a shown in FIG.

In FIG. 6, the light incident areas of the stage 110 and the reference mirror (barrel) 111 are processed so that the mirror surface can sufficiently reflect light. The light of a plurality of frequencies emitted from the light source unit 205 is separated by the light separating element 94, and the light transmitted through the light separating element 94 enters the measuring unit 204d. The two lights emitted from the measurement unit 204d are reflected by the stage 110 and the reference mirror (barrel) 111, and return to the measurement unit 204d again, so that the relative geometry of the stage 110 and the reference mirror (barrel) 111 in the y direction is obtained. Is obtained.

The light reflected by the light separating element 94 is reflected by the reflecting mirror 4.
6 and enters the measuring unit 204e via the reflecting mirror 47.
The two lights emitted from the measuring unit 204e are
Is reflected by the reference mirror (barrel) 111, and is again measured by the measuring unit 204e.
The relative geometric displacement of the stage 110 and the reference mirror (barrel) 111 in the x direction can be obtained by returning to the above. In such two-axis measurement as well, the refractive index fluctuation of air on the measurement optical path of each axis can be measured and corrected.
10 can be positioned with high precision.
The measuring units 204d and 204e have the same configuration as the measuring unit 204a.

The present invention is not limited to the above embodiment, but can be variously modified. For example, in the above embodiment, the measurement example in which the light from the light source unit is separated into two axes is described. However, the light from the light source unit is separated into more axes to perform highly accurate displacement measurement, angle measurement, and the like. You can also. In addition, although the measurement optical path and the reference optical path of the interferometer have been described in the case of a single path, the present invention is not limited to this, and can be applied to an optical path of a double path.

[0081]

As described above, according to the present invention, any displacement of the reflecting mirror which occurs during movement is measured, and an error component due to a change in the refractive index in the optical path and an error component due to a change in the inclination of the moving mirror. , It is possible to realize highly accurate light wave interference measurement.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a light wave interference measurement device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration of angle measurement in the lightwave interference measurement device according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a schematic configuration of a light wave interference measurement device according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a schematic configuration of angle measurement in a lightwave interference measurement device according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a schematic configuration of angle measurement in a lightwave interference measurement device according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a schematic configuration of a lightwave interference measurement device according to a third embodiment of the present invention.

FIG. 7 is a diagram showing a schematic configuration of a conventional light wave interference measurement device.

[Explanation of symbols]

 1-3 light source 5-7 arithmetic unit 10-12 frequency conversion element 20-23 acousto-optic element 30-36 light receiving element 40-63 reflecting mirror 70-75 polarization separation (coupling) element 80, 81 frequency separation element 92, 93 light Separation element 100 to 102 Wave plate 110 Stage

Claims (4)

[Claims] 1. A light source unit for emitting a length measuring light beam for measuring a displacement of an object and a refractive index fluctuation measuring light beam for measuring a refractive index fluctuation of a gas on an optical path of the length measuring light beam. A first light beam splitter provided in a common optical path of the length measuring light beam and the refractive index fluctuation measuring light beam, and splitting at least the length measuring light beam into a first measuring light path and a first reference light path. An optical element; a first reference light reflecting mirror provided on the first reference light path; a movable reflecting mirror provided movably along at least the first measurement light path; A first light-receiving unit that receives interference light of the length-measuring light beam that has passed through the reference light path and the length-measuring light beam that has passed through the first measuring light path; and at least the refraction that has passed through the first measuring light path. A second light receiving unit for receiving the interference light generated by the rate variation measurement light beam; A second light beam splitting optical element that splits the length measuring light beam and the refractive index fluctuation measurement light beam emitted from the light source unit into a plurality of light beams; and one of the light beams split by the second light beam splitting optical element. A third measurement optical path provided in the optical path and divided into a second measurement optical path from a position different from the first measurement optical path toward the movable reflecting mirror in parallel with the first measurement optical path and a second reference optical path; A light beam splitting optical element, a second reference light reflecting mirror provided on the second reference light path, and the length measuring light beam and the second measurement light path that have passed through the second reference light path. A third light receiving unit that receives the interference light of the passed measurement light beam, and a fourth light reception device that receives at least the interference light generated by the refractive index fluctuation measurement light beam that has passed through the second measurement light path. , And each of the first to fourth light receiving units is obtained. An interferometer for measuring the amount of displacement of the movable reflecting mirror based on a signal generated by the moving reflector. 2. The light wave interference measuring apparatus according to claim 1, wherein the light beam split by the second light beam splitting optical element is provided in another light path, and is different from the first and second measurement light paths. A third measurement optical path from a position parallel to the first measurement optical path toward the movable reflecting mirror, a fourth light beam splitting optical element for splitting into a third reference optical path, A third reference light reflecting mirror provided, and a fifth light-receiving element that receives interference light of the length-measuring light beam that has passed through the third reference light path and the length-measuring light beam that has passed through the third measuring light path. A light wave interference measurement device comprising: 3. The light wave interference measurement device according to claim 1, wherein the light source unit emits light of at least two or more types of frequencies. 4. A light source for emitting a plurality of light beams having different frequencies, a light beam having a first frequency among the light beams emitted from the light source, a measurement optical path having a movable mirror and a fixed mirror. A first splitting optical element for splitting the light beam into a reference light path, and at least a light different from the first frequency passes through at least the measurement light path. An interferometer for measuring a displacement of a mirror, comprising: a plurality of the interferometers; An optical element, wherein an interferometer different from the one of the plurality of interferometers is arranged on an emission side of the light split by the second split optical element. apparatus.